1. Field of the Invention
The present invention relates to a radio communication system in which a radio transmitting apparatus transmits a packet to a radio receiving apparatus using a hybrid automatic repeat request (HARQ) scheme, the radio transmitting apparatus and the radio receiving apparatus.
2. Description of the Related Art
There is conventionally known a radio communication system that performs packet retransmission control using a hybrid automatic repeat request (HARQ) scheme (e.g., P. Frenger, S. Parkvall, and E. Dahlman, “Performance comparison of HARQ with Chase combining and incremental redundancy for HSDPA,” Proc. VTC2001-Fall, October 2001). This HARQ scheme uses HARQ-IR (Incremental Redundancy) mode and HARQ-CC (Chase Combining) mode as predominant operating modes.
In the HARQ-CC mode, a radio transmitting apparatus transmits the same packet including an information bit and an error correction bit at both initial transmission and retransmission. To be more specific, as shown in FIG. 1A, the radio transmitting apparatus transmits a packet including a systematic bit S which is an information bit and a parity bit P1 which is an error correction bit to the radio receiving apparatus at initial transmission and transmits, at retransmission, completely the same packet transmitted at initial transmission. The radio receiving apparatus symbol-combines the packet received at initial transmission with the packet received at retransmission and thereby improves SINR (Signal-to-Interference-plus-Noise Ratio).
On the other hand, in the HARQ-IR mode, the radio transmitting apparatus transmits a packet including an information bit at initial transmission and transmits a packet including an error correction bit at retransmission. To be more specific, as shown in FIG. 1B, the radio transmitting apparatus transmits a packet including only a systematic bit S which is an information bit at initial transmission and transmits a packet including only a parity bit P1 or P2 which is an error correction bit at retransmission. The radio receiving apparatus improves the coding gain by correcting errors of the systematic bit S received at initial transmission using the parity bit P1 or P2 received at retransmission.
Furthermore, there is also known a radio communication system that improves a throughput of the entire system by adaptively selecting an operating mode under the above described HARQ scheme.
For example, US Patent Application Publication No. 2006/0200722 describes a radio communication system that selects a HARQ operating mode based on a HARQ buffer size minimum required for HARQ processing in a radio receiving apparatus. Compared to the HARQ-CC mode, the HARQ-IR mode generally has more complicated HARQ processing in the radio receiving apparatus and has a larger necessary HARQ buffer size. Thus, when the HARQ buffer size required for the HARQ-IR mode does not exceed the HARQ buffer size of the radio receiving apparatus, the radio communication system of US Patent Application Publication No. 2006/0200722 applies the HARQ-IR mode. On the other hand, when the HARQ buffer size required for the HARQ-IR mode exceeds the HARQ buffer size of the radio receiving apparatus and the HARQ buffer size required for the HARQ-CC mode does not exceed the HARQ buffer size of the radio receiving apparatus, the radio communication system applies the HARQ-CC mode.
Furthermore, Japanese Patent Application Laid-Open No. 2004-253828 describes a radio communication system that selects a HARQ operating mode based on channel quality at initial transmission. To be more specific, the radio communication system of Japanese Patent Application Laid-Open No. 2004-253828 selects the HARQ-IR mode when the channel quality at initial transmission is better than a predetermined threshold and selects the HARQ-CC mode when the channel quality at initial transmission is poorer than the predetermined threshold.
However, the radio communication system of US Patent Application Publication No. 2006/0200722 or Japanese Patent Application Laid-Open No. 2004-253828 may not be able to select an appropriate HARQ operating mode and has a problem that its effect of improving the throughput of the entire system is low.
To be more specific, in the radio communication system of US Patent Application Publication No. 2006/0200722, if a sufficiently large HARQ buffer size is provided for the radio receiving apparatus, the HARQ-CC mode is not selected but only the HARQ-IR mode is selected. With the development of hardware in recent years, a large size HARQ buffer is easily available, and so the HARQ buffer size is not very important. Therefore, in the radio communication system of US Patent Application Publication No. 2006/0200722, the HARQ-IR mode is selected even in the case where the HARQ-CC mode is preferred, thus the effect of improving the throughput of the entire system is low.
Furthermore, in the radio communication system of Japanese Patent Application Laid-Open No. 2004-253828, when channel quality at initial transmission is good, the HARQ-IR mode is selected according to channel quality at initial transmission even if the channel quality at retransmission is poor. However, in such a case, since the effect of improving the coding gain by using the parity bit received at retransmission cannot be expected, the effect of improving the coding gain by applying the HARQ-IR mode is small. Furthermore, since signal processing in the HARQ-IR mode is complicated, the HARQ-CC mode is preferably used instead of the HARQ-IR mode in the above described case.
Furthermore, in the radio communication system of Japanese Patent Application Laid-Open No. 2004-253828, when channel quality is poor not only at initial transmission but also at retransmission, the HARQ-CC mode is selected according to the channel quality at initial transmission. However, in such a case, a coding gain in the HARQ-IR mode can be more expected compared to the effect of improving the SINR in the HARQ-CC mode. Thus, in the above described case, it is preferable to apply the HARQ-IR mode instead of the HARQ-CC mode.
Thus, since the radio communication system of Japanese Patent Application Laid-Open No. 2004-253828 does not take a fluctuation in channel quality at initial transmission and at retransmission into consideration, it is not possible to select an appropriate HARQ operating mode and the effect of improving the throughput of the entire system is low.